Synchronous motors with field starting resistor arrangement

ABSTRACT

A synchronous motor is provided with starting resistors connected on each field coil or between pairs of field coils so as to confine the induced field current to only the portion of the field coils nearest the air gap. This results in less flux leakage and increases the contribution to pull-in torque of the field coils near synchronous speed so the machine can be brought into synchronism with a higher load torque than otherwise.

O United States Patent 1151 3,654,503 Whitne [451 A r. 4 1972 [54]SYNCHRONOUS MOTORS WITH FIELD 899,447 11/1906 Field ..310/183 STARTINGRESISTOR ARRANGEMENT 1,607,030 11/1926 wechsel 1,614,092 1/1927 We1chselInventofl Eugene y, Plttsburgh. 1,255,457 11/1914 Murray ..310/183 73 Wl h E P't l Asslgnee fii g gf acme Corporation l t Primary Examiner--.].D. Miller Assistant Examiner-R. Skudy Filed; l 1970 Attorney-A. T.Stratton, F. P. Lyle and Gordon H. Telfer 21 A l.N 75218 I 1 pp 57ABSTRACT 52] u.s.c1. ..3l0/l62 310/183 310/184 A synchmmus is Providedwith Starting "Sims 318/192 nected on each field coil or between pairsof field coils so as to 51 110.01. ..H02k 19/00 confine the inducedfield current only P" the [53] Field of Search 310/162, 163, 164, 165177, field coils nearest the air gap. This results in less flux leakage310/72, 207, 68, 269, 184, 261, 224, 266, 183, 185; nd increases thecontribution to pull-in torque of the field 3 90 192 coils nearsynchronous speed so the machine can be brought into synchronism with ahigher load torque than otherwise. [5 6] 6 Claims, 4 Drawing FiguresUNITED STATES PATENTS T Maggs.....

D.C. E XCITATION Patented April 4, 1972 2 Sheets-Sheet FIG. I

0c EXCITATION LW Y m/fm v w Eh m /m w C i 8% n I e Q U E Patented April4, 1972 2 Sheets-Sheet 2 SYNCHRONOUS SPEEDY TORQUE Means to switchBACKGROUNDOF THE INVENTION l Field of the Invention I This inventionrelates to synchronous motors and, more particularly, to such motors asare provided with field coil starting resistor arrangements.

2. Description of the Prior Art Synchronous motors generally have astator armature core and winding and a rotor with magnetic field polesaround each of which is a field coil. The field coils are seriallyconnected and the terminals thereof, usually made accessible bycollector rings, permit application of DC excitation. Shorted damperbars or windings in the pole faces are provided for starting byinduction motor action with the stator winding to which alternatingvoltage is applied.

The field coils are normally closed through a suitable resistance duringstarting and also provide inductive reaction upon the revolving fieldproduced by the stator currents. The inductive effects produce torquesufficient to bring the motor and its load to within a few percent ofthe synchronous speed. If the speed attained under starting conditionsis high enough, DC excitation can then be applied to the field and themachine will go to full synchronous speed.

The external resistance in the field circuit has been considerednecessary to limit the induced voltage which would otherwise appear atthe field terminals. After starting, this external resistance isdisconnected from the field circuit. This commonly practiced startingmethod is generally successful. In some instances, however, for examplewith large synchronous motors such as those driving pumps, fans or otherloads which cannot be reduced to-a very low value, it would be desirableto have a means available for increasing the contribution of the fieldcoil circuit to the torque of the machine near synchronous speed so asto permit starting with larger loads and attaining a greater percentageof the machine s pull-in torque before application of DC excitation.

SUMMARY OF THE INVENTION This invention provides a means for increasingthe torque contribution of the field circuit in starting a synchronousmotor by providing a resistive connection between portions of the fieldcoils in a manner that the induced field current during the startingperiod is confined substantially within a portion of the field coilsnearest the rotor'surface. There is thus provided improved couplingbetween the rotor and stator. There is less flux leakage because theflux is produced in a space closer to the stator.

The resistive connections may be made in different ways. For example, anindividual resistor may be provided across a number of turns at theoutside of each individual field coil or a resistive connection may bemade between adjacent coils the effect of which is to limit the inducedcurrent as desired.

Only modest structural modifications are required by this inventioncompared with prior art machines. Suitably located taps on the coils forconnection of the resistors are required as well as some means forsecuring the resistors to the rotor. The resistors may be permanentlyaffixed in the field circuit. out the resistors after the startingperiod could be provided if the cost were justified.

With the use of resistors in the field circuit in accordance with thisinvention, it is possible to leave the field or collector terminalsopen-circuited during the starting period. However, with the provisionof higher resistance in the field circuit, an additional resistor couldbe used across the field terminals of higher value than is normallyprovided, or merely a small thermal capacity resistance to protectagainst abnormal field voltages.

While the purposes of this invention are primarily directed toimprovement of torque contribution of the field circuit in starting,particularly at low slip, it will be recognized that the concept can beturned around. In any instance in which it is desired to increase thereactance of the rotor, suitable resistors in the field circuit may beprovided to confine the induced field current during the starting periodto the innermost turns thereof. Such an occasion might occur whenstarting geared loads to reduce the torque pulsations at half speed andwhere starting torques must be kept below a specified maximum.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partial view, largelyschematic, of one embodi ment of the present invention;

FIG. 2 is a partial view of an alternate embodiment of the invention;

FIG. 3 is a partial view of a further alternate embodiment of theinvention; and

FIG. 4 is a set of characteristic curves of speed versus torque forpurposes of discussing the operation of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, there is showna synchronous motor 10 with a stator core 12 and armature winding 14 towhich is provided means for connection with an alternating voltagesource 16. The motor has a rotor 18 within the stator 12 with an air gap19 therebetween. The rotor 18 has a plurality of field poles 20 thereon.Damper windings 22 are provided on each of the poles 20 on a facethereof toward the stator 12. A field coil 24 is provided around each ofthe poles 20, each coil consisting of a plurality of stacked inductiveturns. All the coils 24 are mutually interconnected in series bybridging conductors 26. Terminals 28 of the field coil circuit extend tomeans such as collector rings 30 for connection with external elementsincluding a source 32 of DC excitation.

The machine elements are shown without considerable structural detailbecause the present invention may be applied to machines of priorconstruction with very little modification in terms of physicalarrangement of elements. It will be understood, of course, that both thestator 12 and rotor 18 are substantially cylindrical elements includingarmature windings 14 and field poles 20 around their inner and outercircumferences, respectively.

The number of turns in each of the field coils 24 is determined by theexcitation voltage supply and field excitation requirements of themotor. The number of turns in each coil 24 of the drawing for purposesof illustration is less than that normally employed in synchronousmachines.

In the practice of the present invention, it is not required that anymodification be made of the field coils 24 insofar as their function ofcarrying the DC excitation current is concerned. However, to avoid theinduction of current throughout the entire field coil 24 during thestarting period with attendant flux leakage because of the remoteness ofthe inner turns of the coil, by this invention there is providedresistive connection to the field coils effective to confine the inducedfield current to a number of turns close to the airgap.

In the embodiment of FIG. 1 the resistive connection is made betweenadjacent pairs of coils 24 by resistors 34 that are conductivelyconnected to taps on certain turns of the coils allowing current to flowin each resistor 34 through the outer turns of the two adjacent coils.

FIG. 2 illustrates an alternate embodiment wherein an individualresistor 36 is provided across a portion of the turns of each field coil24. The embodiment of FIG. 1 is preferred because it requires fewerelements.

FIG. 3 illustrates a further alternate embodiment but one which has aconverse effect to those of FIGS. 1 and 2. Here, pairs of coils 24 haveresistive interconnections 38 between them that limit induced currentfiow to a number of innermost turns of each of the field coils, henceincreasing the reactance of the rotor. The concept of introductionwith-in field coil circuits of resistive interconnections to modify theinductive reactance of the coils can thus be variously employed. Of pri-5 mary interest are embodiments such as FIGS. 1 and 2 for reducing rotorreactance.

The exact position of the resistive interconnections influences theimprovement in pull-in torque which can be obtained. The fewer turnsadjacent the airgap 19 that are affected by the resistor 34 in FIG. 1for example, the less flux leakage results. However, those fewer turnsare required to carry more current because the quantity of ampere turnswill remain nearly the same. In general, it is considered thatsubstantial improvement in accordance with this invention can beachieved in instances in which the number of turns to which the inducedfield current is confined is less than half of the total number ofturns. Problems of increased current during the starting period wouldnot be considered detrimental, generally, if the number of affectedturns were as little as about a tenth of the total number of turns.Therefore, for field windings consisting of approximately 40 turns ofcopper, it is generally suitable in accordance with this invention tomake the resistive connections in the manner of FIGS. 1 and 2 to leaveremaining below the resistors a number of turns from about four to about20.

Uniformity of the number of turns affected on each of the field coils ishighly desirable although not essential to the practice of theinvention. In general, it is desirable to provide a uniform distributionto provide uniform forces acting between rotor and stator on variouspoles during the starting period.

The magnitude of the resistance in the resistive interconnection bearssome consideration although those skilled in the art will experience nodifficulty in selecting suitable values. To have the desired effect onthe induced field current, it is necessary that this interconnectionhave a greater resistance than the otherwise present current paththrough the turns of the coils between the same two taps. A higherresistance is desirable to cause a more favorable speed-torquecharacteristic as will be subsequently explained in connection with FIG.4. A consideration on the maximum value of the resistors is whether thefield coil terminals 30 will be left open-circuited or not. If leftopen-circuited, excessive voltage build up across those terminals has tobe avoided because of high voltage between collector rings and/orinadequate circuit breakers for high voltage such as above 2,500 volts.However, if it is desired to employ a resistor 40 across the fieldterminals, as shown in FIG. 1, it will permit higher valued resistors tobe used in the resistive interconnections between the coils. Theresistance magnitude should be made suitable for the particularapplication. If a suitable trimming resistor 40 is employed, the totalresistance among the field coils may be greater. For example, in thecase of 80,000 horsepower synchronous motors, having twelve poles of43-r turns on each with resistive interconnections in accordance withFIG. 1 made turns down from the outside, satisfactory performance,including the avoidance of excessive voltage at the field terminals, canbe provided using resistors between the field coils of about 0.07 ohmseach while using a trimming resistance of about 4 ohms.

Referring now to FIG. 4, a further explanation of the invention will begiven in connection with a speed-torque characteristic curve whereinspeed and torque are presented on the vertical and horizontal axis,respectively, in arbitrary units. A first curve A shows the contributionto torque with increasing speed of the damper windings 22, such torquebeing initially substantial but diminishing when the speed approachessynchronous or full speed. A second curve B illustrates the contributionto torque of the field coils where the arrangement in accordance withthis invention is not employed. characteristically this curve increasesfrom minimal torque, has some anomalous cusps at about half speed, whichare tolerable if the damper windings provide sufficient torque, and

then increases to a peak 42 that is very near synchronous speed. Thispeak 42 may be quite narrow and typically on bigger machines is in therange of about 0.2 percent to 2 percent slip or difference fromsynchronous speed. The machine as a whole sees only the torque combinedfrom both dam ers and field coils, i.e., the sum of curves A and B. On at ird curve C there is shown a modified speed-torque characteristic forthe field coil contribution in embodiments in accordance with thisinvention. This results in somewhat greater total torque but moreimportantly perhaps, is that the peak torque of the curve nearsynchronous speed is increased so as to cover a greater load asevidenced by the load curve D. In accordance with this invention, thetotal torque provided is increased in relation to the pull-in torquewhich is the maximum at which a machine may be brought into synchronism.Calculations with respect to an embodiment as described above indicatean increase of total torque of from about 75 percent of rated torque toabout 83 percent of rated torque.

The trimming resistor 40 provides a means for adjusting the slip atwhich the peak of the field torque occurs (curve C). For example, use ofthe field coils and resistors 34 without the trimming resistor 40 mayproduce maximum field torque at a slip of some fairly substantial size,say 8 percent. With the machine at maximum field torque under thoseconditions, a trimming resistor 40 of suitable magnitude can, ifdesired, be introduced into the circuit to adjust the slip to a smalleramount, say 3 percent; some moderate reduction in the field coil maximumtorque contribution may also result. The selective use of the trimmingresistor thus offers a way to optimize the location of the peak of curveC and the intersection of curves C and A.

Iclaim:

1. A synchronous dynamoelectric machine comprising: inductively relatedmachine elements including a stator and a rotor; a first of said machineelements having an armature winding thereon; a second of said machineelements having a plurality of field poles thereon each having a facetoward said first machine element, a damper winding on each of saidpoles on said face and a field coil around each of said poles, saidfield coils being mutually interconnected and each comprising aplurality of radially stacked turns; and resistive means connected withsaid field coils to limit the number of said turns in which inducedfield current flows when said machine is started.

2. The subject matter of claim 1 wherein: said resistive means comprisesa plurality of resistor elements each connected between an adjacent pairof said field coils.

3. The subject matter of claim 1 wherein: said resistive meanscomprises, on each of said field coils, a single resistor elementconnected across a number of turns thereof.

4. The subject matter of claim 2 wherein said resistor elements areconnected to said filled coils for increasing motor starting torque bylimiting said turns in which induced field current flows when saidmachine is started to a number of turns nearest said first machineelement.

5. The subject matter of claim 2 wherein: said resistor elements areconnected to said field coils for decreasing motor starting torque bylimiting said turns in which induced field current flows when saidmachine is started to a number of turns farthest from said first machineelement.

6. The subject matter of claim 4 wherein: said first machine element issaid stator, said second machine element is said rotor and is positionedwithin said stator; said field coils are serially interconnected; andsaid resistor elements are individually connected between an adjacentpair of said field coils that are mutually interconnected between theirrespective turns that are nearest said other machine element.

1. A synchronous dynamoelectric machine comprising: inductively relatedmachine elements including a stator and a rotor; a first of said machineelements having an armature winding thereon; a second of said machineelements having a plurality of field poles thereon each having a facetoward said first machine element, a damper winding on each of saidpoles on said face and a field coil around each of said poles, saidfield coils being mutually interconnected and each comprising aplurality of radially stacked turns; and resistive means connected withsaid field coils to limit the number of said turns in which inducedfield current flows when said machine is started.
 2. The subject matterof claim 1 wherein: said resistive means comprises a plurality ofresistor elements each connected between an adjacent pair of said fieldcoils.
 3. The subject matter of claim 1 wherein: said resistive meanscomprises, on each of said field coils, a single resistor elementconnected across a number of turns thereof.
 4. The subject matter ofclaim 2 wheRein said resistor elements are connected to said filledcoils for increasing motor starting torque by limiting said turns inwhich induced field current flows when said machine is started to anumber of turns nearest said first machine element.
 5. The subjectmatter of claim 2 wherein: said resistor elements are connected to saidfield coils for decreasing motor starting torque by limiting said turnsin which induced field current flows when said machine is started to anumber of turns farthest from said first machine element.
 6. The subjectmatter of claim 4 wherein: said first machine element is said stator,said second machine element is said rotor and is positioned within saidstator; said field coils are serially interconnected; and said resistorelements are individually connected between an adjacent pair of saidfield coils that are mutually interconnected between their respectiveturns that are nearest said other machine element.